The thermal management of advanced aircraft is becoming increasingly more difficult because of the rising heat loads associated with advanced avionic systems and increased aircraft speeds and capabilities. This is especially true for advanced fighter aircraft that utilize radar systems which generate a relatively high heat load and which need to be cooled to approximately 30.degree. C. for reliability and life cycle cost considerations.
Conventionally, a combination of ram air cooling and the cooling capacity of the fuel flow to the main engines of the aircraft has been utilized to effect the thermal management of these heat loads. Under most operating conditions, the cooling capacity of the fuel delivered from the fuel tank to the combustors of the aircraft main engines is insufficient to absorb the total heat load of the aircraft. This results in an excess heat load that is absorbed by the ram air cooling. However, the ducting for the ram air cooling circuits takes up substantial aircraft volume, thereby decreasing the volume available for fuel and necessitating a larger/heavier aircraft to achieve a desired range for the aircraft. Additionally, there is a significant drag penalty associated with directing the ram air through the ram air scoop and ducting.
It is known to absorb the excess heat with an air cycle cooling system, rather than with ram air cooling. Examples of such systems are the Thermal and Energy Management Module (TEMM) (developed by a team of McDonnell Douglas, Pratt & Whitney, and Allied Signal) and the Integrated Power Package (IPP) (developed by Lockheed Martin). These systems integrate the functions of the environmental control system (ECS), the auxiliary power unit (APU), and the emergency power unit (EPU) into one rotating machine and associated valves and heat exchangers. The rotating machine is essentially a cooling turbine combined with a multi-mode gas turbine engine with a generator output for ground and emergency operation. To provide the cooling function for removing the excess heat load, bleed air from the compressor section of the main engine is used to drive the power turbine of the gas turbine engine which in turn drives the compressor of the gas turbine engine. The compressor provides a compressed air flow to a heat exchanger provided in the fan bypass duct of the main engine. The compressed air flow is cooled in the heat exchanger and then delivered to the cooling turbine which expands the air flow to provide a cold air flow for cooling the aircraft cabin and other components of the aircraft.
By integrating several formerly separate subsystems and eliminating the use of ram air cooling and its associated ducting, the TEMM and IPP systems allow for a lower weight, cost and volume package for an aircraft in comparison to conventional aircraft having non-integrated subsystems and significant ram air cooling. However, because the TEMM and the IPP systems divert a significant amount of bleed air from the compressor section of the main engine to drive the power turbine of the gas turbine engine, these systems consume much higher amounts of bleed air in comparison to conventional systems and can potentially have an adverse impact on conventional engines and the range of the aircraft. For example, the withdrawal of approximately 200 lbs. per minute of bleed air during a high altitude (approximately 50,000 feet) cruise at approximately mach=0.9 corresponds to approximately 900 horsepower of work from the main engine turbines. This requires higher fuel flows and turbine inlet temperatures in order to compensate for the lost air flow through the turbines because they must produce the 900 horsepower without the benefit of expanding the diverted bleed air flow through the turbine stages. This requirement for more fuel to accomplish any given mission undesirable increases the aircraft's takeoff gross weight.
It is also known to employ a vapor cycle cooling system to provide cooling for the radar avionics of the aircraft. This system is utilized on the F22 fighter aircraft wherein a polyalphaolefin coolant, commonly referred to as PAO coolant, flows from the radar avionics to an evaporator of a vapor cycle system where the coolant is cooled and then recirculated to the radar avionics. The compressor of the vapor cycle system is driven by an electric motor and the condenser of the vapor cycle system is cooled using the fuel flow to the engine. Additionally, a PAO/fuel heat exchanger is utilized to provide backup cooling for the radar avionics if the vapor cycle cooling system fails.
Another approach for providing cooling for an aircraft is disclosed in U.S. Pat. No. 5,414,992 to Glickstein. Glickstein discloses a cooling system wherein bleed air from the compressor of the main engine is expanded through a cooling turbine to provide cold air and shaft power for aircraft components. A heat exchanger cooled by fan air flow diverted from the fan section of the main engine is utilized to precool the bleed air prior to expanding the bleed air across the cooling turbine.